1. Field of the Invention
The present invention relates to a CO.sub.2 compressor for compressing the CO.sub.2 (carbon dioxide) refrigerant used in an air-conditioning system or, more particularly, to a lubricant supply unit for the CO.sub.2 compressor.
2. Description of the Related Art
In an air-conditioning system using a fluoride such as HFC134 as a refrigerant, as has been widely used in the prior art, a lubricant supply unit is employed in which a lubricant such as a refrigerating machine oil mixed in advance with the refrigerant is separated from the refrigerant compressed and temporarily held in the discharge chamber of the refrigerant compressor, and the lubricant thus separated is forced by the differential pressure between the discharge pressure and the intake pressure of the compressor itself, or the differential pressure between the discharge pressure or the intake pressure and the pressure intermediate of the two pressures, without using a lubricant pump or the like requiring driving power, thereby supplying the lubricant to the sliding parts or the like of the refrigerant compressor requiring lubrication for forcibly lubricating such parts.
In the lubricant supply unit described above, if the differential pressure between the discharge pressure and the intake pressure used for forcing the lubricant increases with the rotational speed of the compressor, the flow rate of the lubricant sometimes increases more than necessary. Therefore, as described in Japanese Unexamined UM Publication (Kokai) No. 59-119992, for example, a pressure reducing part such as a thin restrictor or a porous material is inserted in an oil path, or the oil path of the lubricant is narrowed and lengthened, to increase the resistance thereof, thereby to suppress the flow rate of the lubricant.
In an air-conditioning system using CO.sub.2 as a refrigerant, the differential pressure between intake pressure and discharge pressure is about five times higher than that for an air-conditioning system using an ordinary refrigerant such as HFC134. For forcible lubrication by forcing the lubricant under the differential pressure between the discharge pressure and the intake pressure of the refrigerant compressor using CO.sub.2 as a refrigerant, it is necessary to reduce the flow rate of the lubricant much more than when an ordinary refrigerant is used. Thus, a restrictor, or a like pressure reducing part, arranged in the oil path is required to be very thin and long.
Fabrication of a thin, long pressure reducing part for use in the lubricant path requires labor for machining, which leads to a high cost. Not only that, foreign matter such as metal dust generated at the time of machining and sometimes remaining attached to the parts constituting a refrigeration cycle, or foreign matter or a highly viscous lump produced when a condensing material mixed, though rarely, in the refrigerant or the lubricant is condensed, can clog the pressure reducing parts such as a thin restrictor formed in the lubricant supply unit and prevents stable lubrication. This can reduce the performance and reliability of the compressor and hence of the air-conditioning system.